In the evolution of organs, skin came first. The discovery that even sponges have a proto-skin shows that the separation of insides from outsides in multicellular animals was key to their evolution.

It has been known since the 1960s that sponges have a distinct outer layer of cells, or epithelium. But because sponges lack the genes involved in expelling molecules, it was assumed that this was not a functional organ. Sally Leys and her team at the University of Alberta in Edmonton, Canada, have now shown otherwise. When they grew flat sponges on thin membranes, with liquid above and below, they found that the epithelium kept some molecules out, sometimes only allowing 0.8 per cent through in 3 hours (PLoS ONE, DOI: 10.1371/journal.pone.0015040).

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Sponges were the first multicellular animals to evolve, so the finding means all complex life has a skin. Leys thinks the organ was vital as it isolated animals' insides from their surroundings. As a result, cells could send chemical signals to each other without interference, setting the stage for complex organs to evolve.

Scott Nichols of the University of California, Berkeley, says the findings hint that sponges were the ancestors of other animals rather than a sister group.